Usually, empty coated pipe is buoyant in sea water, and a concrete weight jacket is added to provide sufficient weight to cause the pipe to sink and lay properly on the sea floor. When laid offshore from a vessel, the pipe is subjected to tension applied to minimize the angle of the sag bend on the sea floor. Tension is applied to the pipe through the weight jacket by "tensioners" or especially designed grips.
In another offshore pipe laying method, the pipe is made up on shore, and then pulled or dragged to its location on the sea floor, or river bed. In this case, tension is applied to the pipe, but resistance to moving is felt by the concrete weight jacket.
As the water depth increases, or the pipe lengths to be laid increases, the longitudinal force or sliding force on the weight jacket increases, and failure can occur by the sliding of the jacket over the pipe. This is particularly the case when the corrosion coat presents a smooth surface as do the relatively new synthetic polymeric coatings such as fusion bonded epoxy or polyethylene.
The following references are pertinent to the invention: Choate, L. C., "Feasibility Study of Applying Concrete Over External Thin Film Submarine Pipe Coatings", Materials Performance, pages 45-46, September, 1975; "Tests Show Tension Machine Effects on Pipe Coatings for Offshore Lines", Pipeline Industry, pages 58-60, March, 1969; U.S. Pat. Nos. 3,955,600; 921,354; 868,785; 1,892,433; 2,102,969; 3,761,557.